The invention relates to the use of a composition based on polyol-treated silicon dioxide as antiblocking agent, to an antiblocking agent and to a process for its production.
Micronized silicic acid gels are used to a large extent as antiblocking agents in polymer films. Synthetic amorphous silica gels have a relatively large specific pore volume (usually called porosity) and accordingly supply a larger number of particles (of a certain size) per gram than products having a smaller porosity (e.g. talc, chalk, natural silica gels such as diatomaceous earth). When these particles, which generally have diameters between 1 and 10 .mu.m (Coulter Counter), are incorporated into polymer films in concentrations in the order of magnitude of 0.1%, they produce microscopic surface deformations which prevent complete contact of film layers lying on one another and make the separation of the film layers from one another (for example in the case of carrier bags) or unwinding of the film rolls easier. This is the "antiblocking" effect. For the above reasons, micronized synthetic silica gels are more effective antiblocking agents than products having a low or no porosity. The specific surface of products is linked to their porosity: a very low porosity (almost zero) corresponds to a very small specific surface (less than 1 m.sup.2 /g).
In many cases, a slip agent is additionally added to the antiblocking agent in polymer films. In most cases the slip agent is a fatty acid amide such as oleic acid amide or erucic acid amide and makes the slipping of the film layers over one another easier (slipping effect). However, the antiblocking agent present reduces the effectiveness of the slip agent because the surface of the antiblocking agent is polar and the polar amide molecules are adsorbed on it. As a result, some of the slip agent is not available on the film surface where it is required for the desired slipping effect. Natural products have a very small surface of 0.1 to 0.5 m.sup.2 /g, compared with synthetic SiO.sub.2 products having a surface of 300 to 600 m.sup.2 /g. They adsorb less slip agent than for example silica gel, but show only a very small antiblocking effect because of the low porosity and the small surface.
The antiblocking effect of synthetic silica gels is almost three times greater than that of products with a small surface, but unfortunately, synthetic silica gels adsorb slip agents. This means that, in order to achieve the desired antiblocking and slipping properties, a polyolefin film has for example to be supplied with 0.3 wt. % of an antiblocking agent having a small surface and 0.1 wt. % slip agent, or with 0.1 wt. % antiblocking agent made from synthetic silicic acid and 0.15 wt. % slip agent. This shows that the effectiveness of the slip agent in the presence of synthetic silicic acid is considerably reduced, i.e. approximately 50% more slip agent is required in order to obtain the same slipping effect or the same low coefficient of friction.
Thus, although the traditional synthetic silicic acids are highly effective antiblocking agents, the adsorption of slip agent represents a problem, because
a) it makes it difficult to predict the effect of the slip agent in the film which can ultimately be achieved, PA1 b) the higher quantity of slip agent increases the costs of film manufacture and PA1 c) the required higher quantity of slip agent increases the extractable quantity of organic constituents, which is of importance as regards approval of the film for the packing of foods. PA1 a particle size of 2 to 8 .mu.m (Coulter Counter), PA1 a specific surface of 150 to 850 m.sup.2 /g and PA1 a specific pore volume of 1.4 to 2.0 ml/g, which has been treated with polyol. PA1 a particle size of 2 to 8 .mu.m, PA1 a specific surface of 150 to 850 m.sup.2 /g and PA1 a specific pore volume of 1.4 to 2.0 ml/g, which has ben treated with ethoxylated pentaerythritol in which the pentaerythritol/ethoxy ratio lies in the range from 1:0.5 to 1:25.
Described in U.S. Pat. No. 4,629,749 is the use of polyethylene glycol (PEG) together with antiblocking agents such as diatomaceous earth for improving the clarity of a blown film. Because of their small specific surface and their small pore volume, diatomaceous earths are not very efficient as antiblocking agents. Neither a slip agent nor the adsorption of slip agents at silicon dioxide is mentioned in the patent. The process described in U.S. Pat. No. 4,629,749, used for the treatment of the silicon dioxide, is very expensive and disadvantageous because of solvent emission. The PEG is preferably dissolved in acetone and mixed with the silicon dioxide. The solvent is then evaporated. Another process described therein (claim 3) consists in firstly melting the polyethylene glycol and then coating it onto the surface of the antiblocking agent. The only described way to achieve this object is given in Example VIII and, otherwise than according to claim 3, involves mixing the polyethylene glycol with the polyethylene resin which already contains antiblocking agent.
Described in FR-A-2 484 428 is the use of ethylene glycol or polyethylene glycol with precipitated silicic acid. The silicic acid is used as filler for rubber. This publication does not relate to supplying polyolefin films with antiblocking agents or the adsorption of slip agents at silicon dioxide.
The Japanese patent application JP 2 055 750 relates to a silicic acid which has either been treated with calcium stearate, paraffin wax or polyolefin wax in order to be used as antiblocking agent for polyolefin films. The silicic acid is itself merely called "finely powdered silicic acid". An adsorption of slip agent or the effect of glycols is not described.
Described in EP-A-0 442 325 is a dulling agent for coatings which is similar to the product of the present application. There is however no technological connection.
Described in EP-A-0 526 117 is a composition which contains synthetic silicon dioxide, a slip agent and an alkylene polyether (polyethylene glycol). The reduced slip agent effect is allegedly raised by adding the alkylene polyethers as "slip boosting agent", with the result that smaller quantities of slip agent suffice for the desired slip agent effect. A silicon dioxide with average pore volume is used (specific pore volume approx. 1.1 ml/g). However, although the compatibility with slip agents has been improved, the thus-treated silicon dioxide does not show adequate compatibility or a high antiblocking effect.